van Holde - Weischet Tutorial: Diffusion vs. Heterogeneity

van Holde - Weischet Tutorial

Boundary Spreading: Diffusion vs. Heterogeneity

During the sedimentation process, each particle is subject to transport due to sedimentation as well as transport due to diffusion. Both effects will be observed in a sedimentation velocity experiment. While transport due to sedimentation provides a very clear signal for sample composition (different components will sediment with different sedimentation coefficients, giving rise to different and separated boundaries), the overlaid diffusion transport will broaden the moving boundary and obliterate the clear separation of boundaries that would be observed if diffusion were not be present. For very large molecules, diffusion is very small and it is possible to distinguish a large number of individual components, since the separation is only obliterated by the diffusion process.

As the sample starts to sediment, components with different sedimentation coefficients start to separate, giving rise to boundary spreading. Each component will then diffuse around its local boundary, giving rise to a sigmoidal boundary shape. If the sedimentation behaviour of different components is very similar, the boundary spreading due to diffusion will also influence the boundary shape of the most similar components. Therefore, it will not be possible to identify multiple components by visual inspection of the boundaries alone. In the worst case scenario, the distribution of sedimentation coefficients will itself be Gaussian distributed, making it virtually impossible to determine if the spreading of the boundary is due to heterogeneity or diffusion.

Consider the three schematic scans below:

Single-
Component
System

Figure 1: A single-component system shown without diffusion (in blue) and with diffusion (in red). The boundary spreads due to diffusion and gives a sigmopidal shape to the boundary.

Multi-
Component
System

Figure 2: A multi-component system shown without diffusion (in blue) and with diffusion (in red). The step functions defining the boundary profiles of each component can lose definition as diffusion increases and overlays on top of sedimentation separation.

Single-
or Multi-
Component
System?

Figure 3: The system shown above gives rise to the red boundary, but is this boundary the result of a single - component system (pink) or many components (blue)? In short, without further analysis it is impossible to tell if the boundary spreading is due to heterogeneity in S or due to diffusion.

www contact: Borries Demeler

This document is part of the UltraScan Software Documentation distribution.

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Last modified on June 12, 1999.

	Single- Component System
Figure 1: A single-component system shown without diffusion (in blue) and with diffusion (in red). The boundary spreads due to diffusion and gives a sigmopidal shape to the boundary.
	Multi- Component System
Figure 2: A multi-component system shown without diffusion (in blue) and with diffusion (in red). The step functions defining the boundary profiles of each component can lose definition as diffusion increases and overlays on top of sedimentation separation.
	Single- or Multi- Component System?
Figure 3: The system shown above gives rise to the red boundary, but is this boundary the result of a single - component system (pink) or many components (blue)? In short, without further analysis it is impossible to tell if the boundary spreading is due to heterogeneity in S or due to diffusion.